The hypothesis is proposed that the high susceptibility of the visual cells of rats to damage by visible light, in comparison to other species, results from an abnormally low stability of the membranes of the outer segment. Rhodopsin bleaching changes the structure of the system and, if excessive, initiates reactions which adversely affect the other parts of the visual cell. Because the visual cells of rats from the mutant strain manifesting visual cell degeneration are excessively sensitive to the damaging action of light, the outer segments of these rats, in comparison to those from other strains and species, have been selected for study. Measurements include: phospholipid content and fatty acid composition, lipid peroxidation, changes of internal structural in response to high oxygen, high temperature, and light, and the influence on these changes of antioxidant, membrane stabilizing and labilizing chemicals.